The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in cellular systems is to change from analog to digital transmission. Equally important is the choice of an effective digital transmission scheme for implementing the next generation of cellular technology. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), (employing low cost, pocket-size, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc.), would be provided by the cellular carriers using the next generation digital cellular system infrastructure and the cellular frequencies. The key feature demanded in these new systems is increased traffic capacity.
Currently, channel access is achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. As illustrated in FIG. 1(a), in FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel reuse.
In TDMA systems, as shown in FIG. 1(b), a channel consists of a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, the problem of interference from different relative signal strength levels is reduced.
Capacity in a TDMA system is increased by compressing the transmission signal into a shorter time slot. As a result, the information must be transmitted at a correspondingly faster burst rate which increases the amount of occupied spectrum proportionally. The frequency bandwidths occupied are thus broader in FIG. 1(b) than in FIG. 1(a).
With FDMA or TDMA systems or hybrid FDMA/TDMA systems, the goal is to ensure that two potentially interfering signals do not occupy the same frequency at the same time. In contrast, Code Division Multiple Access (CDMA) allows signals to overlap in both time and frequency, as illustrated in FIG. 1(c). Thus, all CDMA signals share the same frequency spectrum. In either the frequency or the time domain, the multiple access signals overlap. In principle, the informational datastream to be transmitted is impressed upon a much higher bit rate datastream generated by a pseudorandom code generator. The informational datastream and the high bit rate datastream are multiplied together. This combination of higher bit rate signal with the lower bit rate datastream is called coding or spreading the informational datastream signal. Each informational datastream or channel is allocated a unique spreading code. A plurality of coded information signals are transmitted on radio frequency carrier waves and jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading codes, the corresponding information signal is isolated and decoded.
There are a number of advantages associated with CDMA communication techniques. The capacity limits of CDMA-based cellular systems are projected to be up to twenty times that of existing analog technology as a result of the properties of a wide band CDMA system, such as improved coding gain/modulation density, voice activity gating, sectorization and reuse of the same spectrum in every cell. CDMA is virtually immune to multi-path interference, and eliminates fading and static to enhance performance in urban areas. CDMA transmission of voice by a high bit rate decoder ensures superior, realistic voice quality. CDMA also provides for variable data rates allowing many different grades of voice quality to be offered. The scrambled signal format of CDMA completely eliminates cross talk and makes it very difficult and costly to eavesdrop or track calls, insuring greater privacy for callers and greater immunity from air time fraud. Various aspects of CDMA communications are described in K. Gilhousen et at., "On the Capacity of a Cellular CDMA System", IEEE Trans. on Vehicular Technology vol. 40, pp. 303-312 (May 1991).
In systems optimized for digital data transmission, M-ary digital modulation methods, in which one of M possible signals are transmitted during each signalling interval, are often used because of their improved efficiency. One method commonly used is Quadrature Phase Shift Keying (QPSK) in which two equal-magnitude signals in phase-quadrature are impressed on the carrier wave. Another common method is Offset QPSK (OQPSK) in which the maximum phase transition at any point in the modulated waveform is less than the maximum phase transition in a QPSK waveform. As a result, the composite OQPSK signal can have smaller envelope fluctuations after bandpass filtering, and thus more closely approach the constant envelope desired for such signalling. It will be appreciated that QPSK and OQPSK are forms of Quadrature Amplitude Modulation (QAM). Various aspects of these modulations are set forth in, for example, F. Stremler, Introduction to Communication Systems, 2d ed., pp. 590-596, Addison-Wesley Publishing Co., Reading, Massachusetts (1982); and S. Cronemeyer et at., "MSK and Offset QPSK Modulation", IEEE Trans. on Communications vol. COM-24, pp. 809-820 (Aug. 1976).
An example of a system optimized for digital data transmission is a CDMA system in which the demodulation of quadrature-modulated signals involves comparing the received wave with a theoretical wave modulated with hypothesized data patterns, e.g., a Viterbi demodulator. Another example of such a system is a CDMA system in which a stronger signal is demodulated first and then is subtracted from the received signal before a remaining weaker signal is demodulated, as described in commonly assigned U.S. Pat. No. 5,151,919 and allowed application Ser. No. 07/739,446 now U.S. Pat. No. 5,218,619. Both of these documents are expressly incorporated here by reference.
A typical quadrature modulator takes advantage of the quadrature phases of sine and cosine waves to modulate twice the information rate on the radio carrier wave. For example, the even bits in a digital information datastream can be modulated on the cosine wave, and the odd bits in the digital information datastream can be modulated on the sine wave. Errors can arise in quadrature modulators whenever the phases of the cosine and sine waves are not quite 90.degree. apart, or whenever the amplitudes of the sine and cosine waves are not quite equal, or whenever there is residual carrier leakage when the modulating wave is supposedly zero, as well as for other reasons. Of course, the accuracy with which the quadrature modulation matches a synthesized, theoretical wave modulated with hypothesized data or with already received data is important in the above-described communication systems. The accuracy of quadrature modulators has been maintained conventionally by a combination of ensuring good matching between components and by making trimming adjustments to reduce residual mismatch errors.
A conventional quadrature modulator, shown in FIG. 2, includes an "in-phase" or I modulator 101, a "quadrature" or Q modulator 102, and a phase-splitting network 103 for supplying the double-sideband, suppressed carrier modulators 101,102 with cosine and sine carrier frequency signals, respectively. Ideally, the signals provided by the network 103 are cos(.omega.t) and sin(.omega.t), where .omega. is the carrier signal's angular frequency. Also shown in FIG. 2 are an I and Q modulation generator 104 for supplying I and Q modulation signals, a combination network 105 for adding the outputs of the I modulator 101 and the Q modulator 102, and trim potentiometers 106, 107 for carrier balance/d.c. offset adjustments for the I and Q signals, respectively. Additional trim potentiometers 108, 109 for amplitude matching the I and Q signals, respectively, are also shown in FIG. 2. The phase-splitting network 103 may also be adjustable, as indicated by the diagonal arrow, to achieve as nearly as possible the desired 90.degree. phase difference between the sine and cosine carder frequency signals.
In practice, if the I modulator 101 and Q modulator 102 are constructed on the same silicon chip by integrated circuit technology, they will be very well matched, possibly eliminating the need for the amplitude adjustment potentiometers 108, 109. Also in some cases, the purposes of the phase-splitting network 103 can be achieved by starting with a signal having a frequency of 4.omega., i.e., four times the desired carrier frequency .omega., and using the 4.omega.-signal to clock a digital logic divide-by-four circuit that produces the bit patterns: EQU 0011001100110011 . . .
and EQU 0110011001100110 . . . ,
which can be recognized as square waves of frequency 1/4 the 4.omega. bitrate having phases exactly 1/4 of a period (90.degree.) apart. It is usually acceptable to drive the I and Q modulators with square-wave carrier signals instead of with sinusoidal signals. Such a digital method of producing 90.degree.-phased signals can be practical for frequencies ranging up to hundreds of megahertz, but at higher frequencies slight differences in speed of loading of the logic circuits can once more become a significant source of modulator error.
The carrier balance and/or d.c. offset adjustments try to ensure that, when the modulation generator 104 produces a zero signal level on its I and Q outputs, the corresponding output at the carrier frequency of the I and Q modulators is also zero. In essence, this requires the I modulator 101 to produce a zero cosine signal output for a zero I modulation and the Q modulator to produce a zero sine signal output for a zero Q modulation. It is well known that an I modulator imbalance can actually produce a sine signal when the cosine signal is zero, and a Q modulator imbalance can produce a cosine signal when the sine signal is zero. Accordingly, a small cosine leakage from the I modulator is sometimes desired to balance a cosine leakage from the Q modulator, and a small sine leakage from the Q modulator is sometimes desired to balance a sine leakage from the I modulator. With the two adjustment potentiometers 106, 107, however, carrier balance is more readily achievable.
Other sources of modulation inaccuracies are non-linearity in the modulators 101,102 and non-linearity in the modulation generator 104. The generator 104 often produces precursors of the I and Q modulation signals numerically by means of a digital signal processor, and then converts the precursors to analog modulating signals be means of digital-to-analog (D/A) converters. Mismatches between the I-signal D/A converter and the Q-signal D/A converter or in the anti-aliasing filters thereafter are a further source of modulation error. In some cases, the digital signal processor computes a pre-distortion of the modulating signal using an inverse of the non-linear transfer functions of the modulators 101,102 in order to compensate for modulator non-linearity. Techniques for simplifying the D/A conversion and subsequent anti-aliasing filtering by use of highly oversampled delta-modulation are also known, and lead to some reduction of the aforementioned modulation errors. One such technique is described in commonly assigned U.S. patent application No. 07/967,027 entitled "Multi-Mode Signal Processing", which is expressly incorporated here by reference.
U.S. Pat. No. 4,985,688 to Nagata discloses a modulation system in which an amplified, modulated output signal is fed back to a quadrature demodulator. The signal is demodulated and compared to a threshold value. A control signal is generated based on this comparison to adjust the system for nonlinearities of the amplifier that is connected to the modulator. When the threshold is exceeded, the normal modulation is apparently interrupted and replaced by a signal of 1/N-th the frequency or data rate. The Nagata patent also describes how to determine the instants at which the output of the quadrature demodulator should be sampled by use of a differentiator, divider circuit, and clock controlling means.
The Nagata patent's device may also be described as an adaptive, self-learning predistortion arrangement. The stated purpose of the Nagata patent is to inverse-predistort the input to a quadrature modulator such that the output after a distorting power amplifier is correct. On the other hand, the Nagata patent's device can hardly correct for errors in a quadrature modulator because it uses a quadrature demodulator to assess errors, and as described above the demodulator is likely to suffer from the same type of errors as the modulator. After all, if one could make a perfect demodulator, one would simply use it as a perfect modulator.
U.S. Pat. No. 4,581,749 to Carney et al. discloses a frequency modulation device usable in a mobile communication system. A feedback loop provides control of angle modulation error by comparing the modulated deviation amount with a predetermined deviation value. The automatic modulation error correction system described is for transmitters using pure angle modulation, specifically binary continuous-phase frequency shift keying (CPFSK).
In the system described in the Carney patent, an exact modulation index is generated by digitally switching the frequency between two exact values. Nevertheless, such a modulation is not used for transmission because the transitions are not filtered. The transmit waveform uses shaped one-zero transitions to contain the spectrum, and when a sufficient number of like bits occur in a row the frequency deviation of the shaped modulation should approach the same value as the unshaped modulation. Occurrences of such strings of like bits are detected and a comparison is made when they occur, the result being used in a feedback loop to adjust the modulation index. Thus, the Carney patent assesses errors only when the modulation is a long-enough string of ones or zeroes.
U.S. Pat. No. 5,020,076 to Cahill et at. describes switching between analog FM modulating a carrier signal source in the conventional way, and modulating it using a quadrature modulator. The quadrature modulator is left in the circuit when conventional FM is carried out, and the I and/or Q modulation signal is just set to a constant so that the quadrature modulator passes the FM signal straight through.
U.S. Pat. No. 4,856,025 to Takai describes a transmit diversity implementation for improving digital radio communication. A special waveform and special receiver are used, but the special receiver does not assess the accuracy of the transmitter modulation to provide information to a modulation correction system.
It will be appreciated from the foregoing that high modulation accuracy has heretofore been achieved by good design practices combined with specific, fixed, once-and-for-all adjustments that can compensate for fixed, immutable imperfections. It would be highly desirable to be able, continuously and interactively, to adjust and compensate for mutable modulation inaccuracies and errors as well.